Grease Pencil strokes don't respect world space
OK, after throwing the question out to Twitter, it turns out it is a bug, wherein, (as of v2.91.0) GP objects who's origin points are too close to each other can't seem to sort their display and render orders properly.
The temporary fix is to move the offending layer's origin point further out into the world space. This isn't optimal because now it has a &quot;false&quot; world space coordinate, and now you can't really rotate the GP object properly. But it's a solution for now, as long as your layers rotations don't need to animate.